This invention relates to apparatus for correcting displacement of a movable member or a supporting member for slidably supporting the movable member, of a machine tool, for example, and more particularly to apparatus for optimumly correcting displacement of the movable member in accordance with the position thereof after it has been moved to that position (hereinafter, this position is called a moved position).
Such machine tools as a horizontal boring machine, a horizontal milling machine, a draw cut shaper and a diesinking machine are provided with a movable member, for example a ram, and a supporting member slidably supporting the same, for example a spindle head. In such machine tools, such displacements occur caused by deflection of the ram when the ram is advanced or a large tool or attachment is mounted on the ram, or by inclination of the spindle head due to change of the position of the center of gravity of the spindle head.
To attain high accuracies of the machining of the machine tool, it is necessary to correct or compensate for these displacements and various types of displacement correction means have been used. For example, Japanese Laid-Open Patent Specification No. 2185/1965 discloses a balancing apparatus provided with means for compensating for the variation in the position of the center of gravity wherein the oil pressure supplied to an oil pressure cylinder provided for a balance weight is varied in accordance with the forward movement of the ram for the purpose of automatically preventing variation in the moment caused by the variation of the position of the center of gravity of the spindle head. Furthermore, Japanese Laid-Open Patent Specification No. 38827/1975 discloses ram stress compensating apparatus wherein a tension bar is embedded in the upper portion of the ram and the rear end of the tension bar is pulled by an oil pressure cylinder assembly so as to compensate for the deflection of the ram. According to another approach, oil pockets are formed at the front and rear ends of the lower surface of the ram sliding surface of the spindle head and pressurized oil is admitted into these pockets for always maintaining the gaps between the front and rear ends and the ram sliding surface at a constant value so as to correct the inclination of the ram in accordance with the difference in the gaps. According to still another approach, oil pressure cylinders are provided for the upper and lower portions of the front end of the spindle head for correcting the deflection of the front lower portion of the spindle head.
In these displacement correction apparatus, the oil pressure of a pump is adjusted in accordance with the quantity of advance of the ram by means of a pressure regulating valve so as to correct the displacement of the spindle head or ram by supplying pressure regulated oil to oil pressure cylinders. As the pressure adjusting valve has been used a balanced piston type valve including a pilot valve capable of independently adjusting the oil pressure (secondary pressure) supplied to the oil pressure cylinder from two directions. More particularly, adjusting screws are provided for both ends of a pilot valve, one of the adjusting screws being coupled with a ram feed shaft through a gear mechanism so as to rotate the adjusting screw in an interlocked relation with the movement of the ram, thus automatically adjusting the secondary pressure, while the other adjusting screw being connected directly to the adjusting dial of the spindle head so that when a tool or an attachment is mounted on the ram or spindle shaft, the adjusting screws are rotated with an adjusting dial, thereby preadjusting the initial pressure by an amount equal to the variation in the position of the center of gravity of the spindle head caused by the weight thereof.
As above described, according to the prior art displacement correcting apparatus, balanced piston type pressure adjusting valves are used and an adjusting screw provided for one end of a pilot valve is rotated at a predetermined ratio for adjusting the oil pressure. With such method of adjusting, the oil pressure can be adjusted only at a certain constant ratio in accordance with the amount of advance of the ram and it has been impossible to change the percentage of adjustment on the way of advancing the ram. More particularly, the percentage of pressure adjustment (pressure characteristic) of the balanced piston type pressure adjusting valve is principally determined by the force of a spring so that it is necessary to exchange the spring in order to change the characteristic.
Generally speaking, the amount of advance of the ram is not always related, at a definite ratio, to the deflection of the ram or the variation in the position of the center of gravity of the spindle head and as the amount of advance of the ram increases, the amount of displacement tends to increase rapidly. This tendency is particularly remarkable when a tool or an attachment of a large weight is mounted on the front end of the ram or spindle shaft. Accordingly, with the prior art displacement correction apparatus utilizing the balanced piston type pressure adjusting valve which can adjust the pressure only at a constant ratio, it is impossible to provide an optimum displacement adjustment at all advanced positions of the ram. In other words, satisfactory correction can be made only at a specific position or range. Consequently, in a large floor type boring machine, a machine tool mounted with a large attachment or a machine tool having a large stroke of the ram, it has been difficult to obtain a high degree of machining accuracy.
Moreover, it has been necessary to manually adjust the pressure adjusting valve whenever additional load varies as in the case of exchanging the attachment. This not only requires troublesome and time consuming adjustments but also requires assembling while a mechanical coupling adapted to operate the pressure adjusting valve is being adjusted, thus increasing the number of steps required for assembling the displacement correction apparatus.